This invention relates generally to lighting and more particularly, to ceramic discharge chambers for a discharge lamp, such as a ceramic metal halide lamp or a high pressure sodium discharge lamp.
The present invention relates generally to lighting, and more specifically, to a ceramic arc chamber for a discharge lamp, such as a ceramic metal halide lamp. This invention relates particularly to a method of manufacturing ceramic arc chambers, and more particularly, to a method for forming ceramic arc chambers.
Discharge lamps produce light by ionizing a fill such as a mixture of metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill, also known as a xe2x80x9cdosexe2x80x9d, emits a desired spectral energy distribution in response to being excited by the electric arc.
Previously, the discharge chamber in a discharge lamp was formed from a vitreous material such as fused quartz, which was shaped into a desired chamber geometry after being heated to a softened state. Fused quartz, however, has certain disadvantages which arise from its reactive properties at high operating temperatures. For example, at temperatures greater than about 950 to 1,000xc2x0 C., the halide fill reacts with the glass to produce silicates and silicon halide, reducing the fill constituents. Elevated temperatures also cause sodium to permeate through the quartz wall. These fill depletions cause color shift over time, which reduces the useful life of the lamp.
Ceramic discharge chambers were developed to operate at high temperatures for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. U.S. Pat. Nos. 4,285,732 and 5,725,827, for example, disclose translucent polycrystalline sintered bodies where visible wavelength radiation is sufficiently able to pass through to make the body useful for use as an arc tube.
Typically, ceramic discharge chambers are constructed from a number of parts extruded or die pressed from a ceramic powder and then sintered together. For example, referring now to European Patent Application No. 0587238, five ceramic parts are used to construct the discharge chamber of a metal halide lamp. Two end plugs with a central bore are fabricated by die pressing a mixture of a ceramic powder and inorganic binder. A central cylinder and the two legs are produced by extruding a ceramic powder/binder mixture through a die. After forming the part, it is typically air sintered between 900-1400xc2x0 C. to remove organic processing aids. Assembly of the discharge chamber requires tacking of the legs to the cylinder plugs, and the end plugs into the end of the central cylinder. This assembly is then sintered to form joints which are bonded by controlled shrinkage of the individual parts. Obviously, a simplified form of the product would be achieved by the reduction in the number of components separately formed. Moreover, the step of properly joining the compounds is time consuming, expensive and a potential point of failure.
Typically, ceramic discharge chambers are constructed from a number of parts extruded or die pressed from a ceramic powder. For example, end plugs with the central bore may be fabricated by die pressing a mixture comprising a ceramic powder and an organic binder. A central cylinder, and the two legs may be produced by extruding a ceramic powder/binder mixture through a die. Assembly of the discharge chamber involves the placement and tacking of the legs to the end plugs and the end plugs into the ends of the central cylinder. This final assembly is then sintered to form four centered joints which are bonded by controlled shrinkage of the individual parts. The conventional ceramic discharge chamber method of construction has a number of disadvantages. For example, the number of component parts is relatively large and introduces the corresponding number of opportunities for variation and defects. Also, the conventional discharge chamber includes four bonding regions, each of which introduces an opportunity for lamp failure by leakage of the fill material if the bond if formed improperly. Each bonding area also introduces a region of relative weakness, so that even if the bond is formed properly, the bond may break during handling or be damaged enough in handling to induce failure in operation.
Another disadvantage relates to the precision with which the parts can be assembled and the resulting effect in the light quality. It is known that the light quality is dependent to a substantial extent on the voltage across the electrode gap, which in turn is dependent on the size of the gap consistently achieve the gap size within an acceptable tolerance without significant effort devoted to optimizing the manufacturing process. Accordingly, it would be desirable to minimize the component parts necessary to manufacture the ceramic arc chamber. However, divergent shrinkage rates of variously shaped components and other factors have limited the ability to manufacture in a more efficient manner.
According to an exemplary embodiment of the invention, a method is provided for making a component of a ceramic discharge chamber by forming a ceramic composition including a ceramic powder, a binder, and a grain growth inhibitor. The ceramic powder has a tap density of greater than about 1.0 gm per cc. The ceramic composition is then die pressed to form the desired ceramic discharge chamber preform component. The preform component can then be assembled with additional preform components into a presintered discharge chamber and sintered to join the components via controlled shrinkage.
The preform components of the present invention can facilitate the assembly of an arc discharge chamber from fewer components than demonstrated previously. Moreover, the present method allows intricate shaped components to be die pressed. More specifically, the end cap members can be die pressed in the shape of the disk shaped body and leg extension. This clearly provides a manufacturing simplicity versus conventional manufacturing steps which included the extrusion of leg portions which must then be secured to end caps or extensive machining.
Exemplary embodiments of the invention can be used to improve the performance of various types of lamps, such as metal halide metals, high pressure mercury vapor lamps, and high pressure sodium vapor lamps.